Vitreous enamel resistance material and resistor made therefrom



March 31, 1970 c. Y. n. HUANG ETAL 3,

VITREQUS ENAMEL RESISTANCE MATERIAL-AND RESISTOR MADE THEREFRQM FiledNOV. 29, 1967 /4 RES/STANCE /5 MATERIAL ME TA 1. BUR/D E PART/C155 /2CERAMIC SUBSTRATE 0 IN VEN TORS- CORNEL/US K0. HUANG KENNETH M. MERZ ATTORN' Y United States Patent 3,503,801 VITREOUS ENAMEL RESISTANCEMATERIAL AND RESISTOR MADE THEREFROM Cornelius Y. D. Huang, Bala Cynwyd,and Kenneth M.

Merz, Malvern, Pa., assignors to TRW Inc., a corporation of Ohio FiledNov. 29, 1967, Ser. N 0. 686,592 Int. Cl. H01c 7/00 US. Cl. 117-221 2Claims ABSTRACT OF THE DISCLOSURE A vitreous enamel resistance materialcomprising a mixture of a vitreous glass frit and fine particles of ametal boride of the transition elements of Groups IV, V and VI of theperiodic chart. The metal boride may be chromium boride (CrB zirconiumboride (ZrB molybdenum boride (MoB tantalum boride (TaB or titaniumboride (TiBg). The metal boride is present in the vitreous enamelresistance material in the proportions of, by weight, 10% to 35% forchromium boride, to 50% for zirconium boride, 20% to 50% for molybdenumboride, 35% to 65% for tantalum boride and 20% to 50% for titaniumboride. An electrical resistor is made with the vitreous enamel resistormaterial of the present invention by coating a ceramic substrate withthe vitreous enamel resistance material and firing the coated substrateat a temperature sufiicient to melt the glass frit of the vitreousenamel resistance material. Upon cooling, the glass hardens so that theresultant resistor comprises the substrate having on the surface thereofa film of glass with the metal boride particles embedded in anddispersed throughout the glass film.

BACKGROUND A type of electrical resistance material which has recentlycome into commercial use is a vitreous enamel resistance material whichcomprises a mixture of a glass frit and finely divided particles of anelectrical conductive material. The vitreous enamel resistance materialis coated on the surface of a substrate of an electrical insulatingmaterial, usually a ceramic, and fired to melt the glass frit. Whencooled, there is provided a film of glass having the conductiveparticles dispersed therein. Terminations are connected to the film topermit the resultant resistor to be connected in the desired circuit.

The materials which have been generally used for the conductiveparticles are the noble metals. Although the noble metals providevitreous enamel resistance materials which have satisfactory electricalcharacteristics, they have the disadvantage that they are expensive.Thus, the resistors made from the vitreous enamel resistance materialscontaining the noble metals are expensive to manufacture. Therefore, itwould be desirable to have a vitreous enamel electrical resistancematerial which utilizes a relatively inexpensive conductive material soas to provide an electrical resistor which is relatively inexpensive tomanufacture. In addition, the conductive material used must be capableof providing a resistance material having a wide range of resistancevalues and which has relatively good electrical characteristics over theentire range of the resistance values. Such electrical characteristicsinclude temperature coefficient of resistance, voltage coefficient,stability under load, etc.

SUMMARY It is an object of the present invention to provide a novelvitreous enamel resistance material utilizing a relatively inexpensiveconductive material.-

It is another object of the present invention to provide "Ice anelectrical resistor utilizing a novel vitreous enamel resistancematerial.

It is a further object of the present invention to provide a vitreousenamel electrical resistor having a relatively wide range of resistancevalues, which has relatively good electrical characteristics over theentire range of resistance values and which is relatively inexpensive tomanufacture.

It is a still further object of the present invention to provide avitreous enamel resistance material comprising a mixture of a glass fritand finely divided particles of a metal boride selected from the groupconsisting of molybdenum boride, titanium boride, zirconium boride,chromium boride and tantalum boride.

Other objects will appear hereinafter.

The invention accordingly comprises a composition of matter and productformed therewith possessing the characteristics, properties and relationof constituents which will be exemplified in the composition hereinafterdescribed, and the scope of the invention will be indicated in theclaims.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a cross-sectional view,on a highly exaggerated scale, of a resistor produced in accordance withthe present invention.

DESCRIPTION OF INVENTION In general, the vitreous enamel resistancematerial of the present invention comprises a mixture of a vitreousglass frit and fine particles of a metal boride of the transitionelements of Groups IV, V and VI of the periodic chart. The metal boridecan be molybdenum (MoB titanium boride (TiB zirconium boride (ZrB chr0-mium boride (CrB or tantalum boride (TaB In the vitreous enamelresistance material of the present invention, the metal boride of theabove-stated group is present in the proportion of, by weight, 20% to50% for molybdenum boride, 20% to 50% for titanium boride, 20% to 50%for zirconium boride, 10% to 35% for chromium boride, and 35% to 65 fortantalum boride.

The glass frit used in the resistance material of the present inventionmay be of any well-known composition which has a melting temperaturebelow that of the refractory metal boride. The glass frits mostpreferably used are the borosilicate frits, such as lead borosilicatefrit, bismuth, cadmium, barium, calcium or other alkaline earthborosilicate frits. The preparation of such glass frits is well-knownand consists, for example, in melting together the constituents of theglass in the form of the oxides of the constituents, and pouring suchmolten composition into water to form the frit. The batch ingredientsmay, of course, be any compound that will yield the desired oxides underthe usual conditions of frit production. For example, boric oxide willbe obtained from boric acid, silicon dioxide will be produced fromflint, barium oxide will be produced from barium carbonate, etc. Theglass is preferably milled in a ball-mill with water to reduce theparticle size of the frit and to obtain a frit of substantially uniformsize.

To make the resistance material of the present invention, the glass fritand refractory metal boride are broken down, such as by ball-milling, toa substantially uniform particle size. An average particle size ofbetween 1 to 2 microns has been found to be preferable. The glass fritand refractory metal boride powder are thoroughly mixed together, suchas by ball-milling in water or an organic medium, such as butyl carbitolacetate or a mixture of butyl carbitol acetate and toluol. The mixtureis then adjusted to the proper viscosity for the desired manner ofapplying the resistance material to a substrate by either adding orremoving the liquid medium of the material.

To make a resistor with the resistance material of the presentinvention, the resistance material is applied to a uniform thickness onthe surface of a substrate. The substrate may be a body of any materialwhich can withstand the firing temperature of the resistance materialcomposition. The substrate is generally a body of a ceramic, such asglass, porcelain, refractory, barium titinate, or the like. Theresistance material may be applied on the substrate by brushing,dipping, spraying or screen stencil application; The substrate with theresistance material coating is then fired in a conventional furnace at atemperature at which the glass frit becomes molten. For resistancematerials of the present invention, it has been found preferable to firethe coated substrate in an inert atmosphere, such as argon, helium,nitrogen or a mixture of nitrogen and hydrogen, to achieve a resistor ofbetter stability. When the coated substrate is cooled, the vitreousenamel hardens to bond the resistance material to the substrate.

As shown in the drawing, the resultant resistor of the present inventionis generally designated as Resistor 10 comprises the ceramic substrate12 having a layer 14 of the resistance material of the present inventioncoated and fixed thereon. The resistance material layer 14 comprises theglass 16 and the finely divided particles 18 of the metal borideembedded within and dispersed throughout the glass 16.

EXAMPLE I A plurality of resistance materials of the present inventionwere made in which the conductive material was molybdenum boride in thevarious amounts shown in Table I and the glass frit was a barium,titanium, aluminum borosilicate glass. Each of the resistance materialswas'made by mixing together the glass frit and molybdenum borideparticles in a ball-mill in butyl carbitol acetate. Resistors were madewith each of the resistance materials by coating cylindrical ceramicbodies with the resistance material and firing the coated ceramic bodiesin a furnace at approximately 900 C for thirty minutes. The resistorswere fired in a nitrogen atmosphere. A number of resistors of each ofthe compositions were made, and the average resistance values andtemperature coefiicient of resistance of the resulting resistors of.each group are shown in Table I.

A plurality of resistance materials of the present invention were madein which the conductive material was zirconium boride in the variousamounts shown in Table II and the glass frit was a barium, titanium,aluminum borosilicate glass. Each of the resistance materials was madein the same manner as the resistance materials of Example I, andresistors were made with each of the resistance materials in the samemanner as described in Example I. The resistors were fired at 1050" C.in a nitrogen atmosphere for thirty minutes and the average resistancevalues and temperature coefficient of resistance for each group of theresultant resistors are indicated in Table 11.

TABLE II Temperature Coef. of

Zirconium resistance (percent per C.)

boride Glass frit (percent by (percent by Resistance +25 C. to +25 C. toweight) weight) (ohms/El) 150 C. 55 C.

EXAMPLE III A plurality of resistance materials of the present inventionwere made in which the conductive material was chromium boride in thevarious amounts shown in Table III and the glass frit was a barium,titanium, aluminum borosilicate glass. Each of the resistance materialswas made in the same manner as the resistance materials of Example I,and resistors were made with each of the resistance materials in thesame manner as described in Example I. The resistors were fired at 850C. in a nitrogen atmosphere for thirty minutes and the averageresistance values and temperature coefiicient of resistance for eachgroup of the resultant resistors are indicated in Table III.

A plurality of resistance materials of the present invention were madein which the conductive material was tantalum boride in the variousamounts shown in Table IV and the glass frit was a barium, titanium,aluminum borosilicate glass. Each of the resistance materials was madein the same manner as the resistance materials of Example I, andresistors were made with each of the resistance materials in the samemanner as described in Example I. The resistors were fired at 1050 C. ina nitrogen atmosphere for thirty minutes and the average resistancevalues and temperature coefficient of resistance for each group of theresultant resistors are indicated in Table IV.

A plurality of resistance materials of the present invention were madein which the conductive material was titanium boride in the variousamounts shown in Table V and the glass frit was a barium, titanium,aluminum borosilicate glass. Each of the resistance materials was madein the same manner as the resistance materials of Example I, andresistors were made with each of the resistance materials in the samemanner as described in Example I. The resistors were fired in a nitrogenatmosphere for thirty minutes with resistors containing 20% and 25%titanium boride being fired at a temperature of 1150 C. and theresistors containing 30% titanium boride being fired at 1100 C. Theaverage resistance values and temperature coefficient of resistors foreach group of the resultant resistors are indicated in Table V.

TABLE V Temperature Coei. of Titanium resistance (percent per C.) borideGlass frit (percent by (percent by Resistznce +25 C. to +25 C. toweight) weight) (ohms/E!) +150 C. 55 C.

It should be understood that the examples of the resistors andresistance materials of the present invention shown in Tables I throughV are given merely to illustrate certain details of the invention andare not to be taken as in any way limiting the invention thereto. Thepresent invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appending claims, ratherthan to the foregoing specification as indicating the scope of theinvention.

We claim:

1. A vitreous enamel resistor composition adapted to be applied to andfired on a substrate to form an electrical resistor comprising a mixtureof a glass frit and a finely divided metal boride selected from thegroup consisting of chromium boride, zirconium boride, molybdenumboride, tantalum boride and titanium boride,

wherein the metal boride is present in the proportions of, by weight,10% to for chromium boride, 20% to for zirconium boride, 20% to 50% formolybdenum boride, 35% to for tantalum boride and 20% to 50% fortitanium boride.

2. A vitreous enamel resistor composition in accordance with claim 1 inwhich the resistor composition is coated and fired on the surface of aceramic body to provide a glass film having the metal boride particlesembedded in and dispersed throughout the glass film.

References Cited UNITED STATES PATENTS 2,822,302 2/1958 McCaughna1l7-221 WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R.

